I suppose it would probably make sense to put down here everything that happens when a Pokémon is transferred from Gen 1 to Bank.
This is all straight from Serebii btw; just to give credit.
With Version 1.2, the ability to transfer from the Virtual Console versions of Pokémon Red, Blue & Yellow appears and with that, the Pokémon will be sent up. As the backend of the Pokémon are a lot different, there are some notable changes. First, the Pokémon will have a unique symbol in their status screen of a GameBoy showing that they are from the Virtual Console games.
The following also happens
- Ability - If a Pokémon has one, it will have its Hidden Ability, else it'll have its standard ability
- Nature - Nature is chosen based on the Experience of the Pokémon in the Generation 1 game
- IVs - A Pokémon will hve a guaranteed 3 maximum IVs. The rest are determined randomly
- Gender - Random
- Effort Values - Erased
- Shiny - Based on the IVs of the Pokémon in Generation 1: 10 in Attack, Special & Speed and Defense a multiple of 2 = Shiny
On natures: When transferring from Pokémon Red, Green, Blue or Yellow, the Pokémon's nature is determined by dividing the experience the Pokémon has by 25. Then, using the remainder of that, you can determine the nature so if the remainder is 10, then it is Timid, or if it's 0, it is Hardy.
Serebii's full guide for Natures (Gen 1 remainder column):
http://www.serebii.net/games/natures.shtml
In addition, something they didn't write is that to differentiate from Pokémon transferred all the way from FRLG, the status screen will say "Seems to have been met for the first time in the Kanto region in the good old days."
Which, btw, is friggin adorable imo